Guilds can either be global, meaning that they can be used on any server (unless the staff have specifically prevented it), or local. On Graal Kingdoms, for example, there is one guild for each Kingdom. Since it would not make sense for them to be used on other servers, they have been defined as local guilds.
Local guilds can be freely created by server staff. However, global guilds must be approved by the Global Guilds Admin, and this will only happen if enough players support their creation. Guilds whose names contain offensive words or copyrighted terms will most likely be rejected. Even if a guild is deemed acceptable, it will not be enabled until at least ten Graalians have supported it through the Guild CP.